Use of circuit breakers is widespread in modern-day residential, commercial and industrial electric systems, and they constitute an indispensable component of such systems toward providing protection against over-current conditions. Various circuit breaker mechanisms have evolved and have been perfected over time on the basis of application-specific factors such as current capacity, response time, and the type of reset (manual or remote) function desired of the circuit breaker.
One type of circuit breaker mechanism employs a thermo-magnetic tripping device to "trip" a latch in response to a specific range of over-current conditions. The tripping action is caused by a significant deflection in a bimetal element which responds to changes in temperature due to resistance heating caused by flow of the circuit's electric current through the bimetal. The bimetal element is typically in the form of a blade and operates in conjunction with a latch so that blade deflection releases the latch after a time delay corresponding to a predetermined over-current threshold in order to "break" the current circuit associated therewith. Additionally, circuit breaker mechanisms of this type often include an electromagnet arrangement which includes a yoke and armature which are attracted to each other to release the latch in the presence of a very high current or short circuit condition.
Occasionally, bimetals used in this type of circuit breaker would be over heated and deflect in the direction of their normal thermal deflection to a position where they became permanently deformed and would not deflect back to their original shape. To overcome this problem, a stop member was placed on the circuit breaker base to prevent the bimetals from deflecting in the direction of their normal thermal deflection to a position past the point where they were permanently deformed.
However, a problem exists in some circuit breakers during very high current short circuit testing. During these tests, high magnetic repulsive forces cause the bimetal to be repelled away from a current carrying terminal and deflected in a direction opposite its normal thermal deflection, or reverse deflection. This reverse deflection causes the bimetal to be permanently deformed, which renders the circuit breaker inoperative because the bimetal can no longer deflect the distance required to release the latch.
An additional problem exists in circuit breakers that operate in ambient temperatures below room temperature, or 24.degree. C. When the ambient temperature drops below 24.degree. C., the bimetal deflects in the direction opposite its normal thermal deflection. If the ambient temperature drops far enough below 24.degree. C., the latch may not be released in the presence of a short circuit condition.
Accordingly, there is a distinct need for an improved circuit breaker which avoids the aforementioned shortcomings. According to the present invention, a novel reverse deflection prevention arrangement is provided for preventing the bimetal from deflecting in the direction opposite its normal thermal deflection.